utils.js_to_json is broken: mishandles commented lines

[israel-lugo]

Tested with youtube-dl 2020.01.15 and 2020.01.01.

>>> from youtube_dl import utils
>>> input = """{
...   foo: "value",
...   // bar: { nested:'x' },
...   bar: { nested:'x' },
...   chaff: "something"
... }"""
>>> print(utils.js_to_json(input))
{
  "foo": "value"
  
  "bar": { "nested":"x" },
  "chaff": "something"
}

Note that the "foo": "value" line has lost its trailing ',' with the transformation (it was there in the input). This is invalid JSON and is causing issue #23707.

[israel-lugo]

Real life example, input:

{
          id: "player_prog",
          fileKey: "/nas2.share/pt/h264/512x384/p6265/p6265_1_201912152233180805.mp4",
          file : "https://ondemand-streaming.rtp.pt/nas2.share/pt/h264/512x384/p6265/index.m3u8?tlm=hls&streams=p6265_1_201912152233180805.mp4.m3u8:1024",
                                  seekBarThumbnailsLoc: '//cdn-images.rtp.pt/multimedia/screenshots/p6265/preview/p6265_1_201912152233180805_preview.vtt',
                      adsType: 'dfp',
          subadUnit: 'Play',
          streamType:"ondemand",
          googleCast: true,
          //extraSettings: { googleCastReceiverAppId:'1A6F2224', skin:'s3',  skinAccentColor: '0073FF'},
          extraSettings: { googleCastReceiverAppId:'1A6F2224'},
          mediaType: "video",
          poster: "https://cdn-images.rtp.pt/EPG/imagens/37684_55429_70561.jpg?v=3&w=1170"
        }

Is getting transformed to:

{
          "id": "player_prog",
          "fileKey": "/nas2.share/pt/h264/512x384/p6265/p6265_1_201912152233180805.mp4",
          "file" : "https://ondemand-streaming.rtp.pt/nas2.share/pt/h264/512x384/p6265/index.m3u8?tlm=hls&streams=p6265_1_201912152233180805.mp4.m3u8:1024",
                                  "seekBarThumbnailsLoc": "//cdn-images.rtp.pt/multimedia/screenshots/p6265/preview/p6265_1_201912152233180805_preview.vtt",
                      "adsType": "dfp",
          "subadUnit": "Play",
          "streamType":"ondemand",
          "googleCast": true
          
          "extraSettings": { "googleCastReceiverAppId":"1A6F2224"},
          "mediaType": "video",
          "poster": "https://cdn-images.rtp.pt/EPG/imagens/37684_55429_70561.jpg?v=3&w=1170"
        }

Note the missing trailing , after "googleCast": true".